This invention relates to a substrate for an information recording medium for use as a recording medium for an information processing apparatus, an information recording medium using the substrate, and a method of producing the substrate.
A magnetic disk is known as one of information recording media. The magnetic disk comprises a substrate and a thin film such as a magnetic layer formed thereon. As the substrate, use has been made of an aluminum substrate or a glass substrate. In recent years, in response to the demand for high-density recording, the glass substrate is used at an increasing ratio because a gap between a magnetic head (which is operable as a recording and/or reproducing head) and the magnetic disk can be small as compared with the aluminum substrate.
Generally, the glass substrate is produced through chemical strengthening in order to increase the strength so that the glass substrate is resistant against a shock when loaded into a magnetic disk drive. Alternatively, the surface of the glass substrate is heat treated to be crystallized so that a crystallized substrate improved in strength is produced. In addition, in order to lower a flying height of the magnetic head as low as possible, the surface of the glass substrate is polished with high precision. Thus, high-density recording is realized.
Not only the glass substrate, the magnetic head is also developed from a thin film head to a magnetoresistive head (MR head), further to a giant (large-sized) magnetoresistive head (GMR head) so as to meet the high-density recording.
However, there arises a problem that, even if the surface roughness Rmax (which is defined as a maximum height representative of a difference between a highest point and a lower point) or Ra (which is representative of a center-line-mean roughness) is reduced by high-precision polishing, the flying height of the magnetic head can not be lowered. The present inventors investigated the cause to find out that a microwaviness present on the surface of the substrate is responsible and that the microwaviness also affects a modulation of the magnetic disk.
Japanese Unexamined Patent Publication (JP-A) No. H08-147688 proposes a glass substrate for a magnetic disk, in which a surface waviness (Wa) and the surface roughness (Ra) satisfy the relationship Wa/Raxe2x89xa60.5 in order to reduce a glide height of the magnetic disk.
However, measurement of the surface waviness described in the above-mentioned publication uses a tracer-type surface roughness tester (Tencor) and is carried out over the length on the order of 100 xcexcm along a straight line in a radial direction of the substrate. Thus, the measurement is performed only in a restricted local area and in a single direction. Thus, the above-mentioned measurement does not fully reflect the surface waviness of the whole substrate.
It is an object of this invention to provide a substrate for an information recording medium and an information recording medium adapted to a high recording density, by adjusting each of a surface waviness (Wa) and a microwaviness (Raxe2x80x2, wa) on the surface of the substrate to a level not exceeding a predetermined range, where each of the surface waviness (Wa) and the microwaviness (Raxe2x80x2, wa) is measured by a measuring method capable of properly expressing the surface waviness (Wa) and the microwaviness (Raxe2x80x2, wa) and has a correlation with a glide height or a modulation of a magnetic disk.
It is another object of this invention to provide a method of producing a substrate for an information recording medium, which is capable of adjusting each of a surface waviness (Wa) and a microwaviness (Raxe2x80x2, wa) on the surface of the substrate to a level not exceeding the predetermined range.
It is still another object of this invention to provide a method of controlling the surface of a substrate for an information recording medium so as to adjust a gliding height of a head slider or a modulation of the information recording medium to a desired value.
This invention has been made in view of the above-mentioned objects and has the following structures.
(Structure 1)
A substrate for an information recording medium, the substrate having a microwaviness average height Raxe2x80x2 not greater than 0.05 microinch as measured by a contactless laser interference technique for measurement points within a measurement region of 50 xcexcmxe2x96xa1-4 mmxe2x96xa1 on a surface of the substrate, the microwaviness average height Raxe2x80x2 being given by:             Ra      xe2x80x2        =                  1        n            ⁢                        ∑                      i            =            1                    n                ⁢                  "LeftBracketingBar"                                    x              ⁢                              xe2x80x83                            ⁢              i                        -                          x              _                                "RightBracketingBar"                      ,
where xi represents a measurement point value of each measurement point, {overscore (x)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 2)
A substrate for an information recording medium, the substrate having a microwaviness period between 2 xcexcm and 4 mm and a microwaviness average height Raxe2x80x2 not greater than 0.05 microinch as measured by a contactless laser interference technique for measurement points in a measurement region on a surface of the substrate, the microwaviness average height Raxe2x80x2 being given by:             Ra      xe2x80x2        =                  1        n            ⁢                        ∑                      i            =            1                    n                ⁢                  "LeftBracketingBar"                                    x              ⁢                              xe2x80x83                            ⁢              i                        -                          x              _                                "RightBracketingBar"                      ,
where xi represents a measurement point value of each measurement point, {overscore (x)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 3)
A substrate for an information recording medium as described in structure 1 to 2, wherein the measurement region is smaller in area than a slider surface of a head slider of a recording and/or reproduction head.
(Structure 4)
A substrate for an information recording medium, the substrate having a disk-like shape, the substrate having a waviness period between 300 xcexcm and 5 mm and a waviness average height Wa of 1.0 nm or less as measured by a contactless laser interference technique for measurement points in a measurement region surrounded by two concentric circles which is spaced from a center of a surface of the disk-shaped substrate by a predetermined distance, the waviness average height Wa being given by:   Wa  =            1      N        ⁢                  ∑                  i          =          1                N            ⁢              "LeftBracketingBar"                              X            ⁢                          xe2x80x83                        ⁢            i                    -                      X            _                          "RightBracketingBar"            
where Xi represents a measurement point value of each measurement point, {overscore (X)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 5)
A substrate for an information recording medium as described in any one of structures 1 through 4, wherein the substrate is a substrate for a magnetic recording medium.
(Structure 6)
A substrate for an information recording medium as described in any one of structures 1 through 5, wherein the substrate is made of a glass.
(Structure 7)
An information recording medium comprising a substrate described in any one of structures 1 through 6 and at least a recording layer formed on the substrate.
(Structure 8)
An information recording medium as described in structure 7, wherein the recording layer is a magnetic layer.
(Structure 9)
A method of producing a glass substrate for an information recording medium, the method comprising the steps of:
preparing a glass substrate having a flatness of 4 xcexcm or less; and
polishing a principal surface of the glass substrate by the use of a soft polisher having a hardness not greater than 80 (Asker-C) and abrasive grains having an average grain size of 1.0 xcexcm or less.
(Structure 10)
A method as described in structure 9, wherein:
the soft polisher has a hardness between 62 and 70 (Asker-C).
(Structure 11)
A method as described in structure 9 or 10, wherein:
the step of preparing the glass substrate is carried out by lapping of a glass material of a disk-like shape.
(Structure 12)
A method as described in any one of structures 9 through 11, wherein:
the polishing step is carried out with a working surface pressure between 40 and 150 g/cm2 applied to the glass substrate during polishing.
(Structure 13)
A method as described in any one of structures 9 through 13, wherein:
the polishing step is carried out by feeding the abrasive grains at a flow rate of 50 cc/sec or more with respect to the glass substrate.
(Structure 14)
A method as described in any one of structures 9 through 13, wherein the glass substrate is a substrate for a magnetic recording medium.
(Structure 15)
A method of producing an information recording medium, the method comprising the step of forming at least a recording layer on a substrate obtained by a method described in any one of structures 9 through 14.
(Structure 16)
A method as described in structure 15, wherein the recording layer is a magnetic layer.
(Structure 17)
A substrate for a magnetic recording medium, the substrate having a predetermined microwaviness average height Raxe2x80x2 determined by:
measuring the microwaviness average height Raxe2x80x2 by the use of a contactless laser interference technique for measurement points in a measurement region on a principal surface of the substrate;
evaluating a modulation of a magnetic recording medium comprising the substrate and at least a magnetic layer formed thereon;
comparing the microwaviness average roughness Raxe2x80x2 and the modulation to obtain a correlation therebetween; and
selecting, with reference to the correlation, the predetermined microwaviness average height Raxe2x80x2 so that the modulation has a desired value; the microwaviness average height Raxe2x80x2 being given by             Ra      xe2x80x2        =                  1        n            ⁢                        ∑                      i            =            1                    n                ⁢                  "LeftBracketingBar"                                    x              ⁢                              xe2x80x83                            ⁢              i                        -                          x              _                                "RightBracketingBar"                      ,
where xi represents a measurement point value of each measurement point, {overscore (x)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 18)
A method of controlling a principal surface of a substrate for a magnetic recording medium, the method comprising the steps of:
measuring a microwaviness average height Raxe2x80x2 by the use of a contactless laser interference technique for measurement points in a measurement region on the principal surface of the substrate;
evaluating a modulation M of a magnetic recording medium comprising the substrate and at least a magnetic layer formed on the principal surface thereof;
comparing the microwaviness average height and the modulation M to obtain a correlation therebetween; and
determining, with reference to the correlation, the microwaviness average height Raxe2x80x2 of the substrate so that the modulation has a desired value;
the microwaviness average height Raxe2x80x2 being given by:             Ra      xe2x80x2        =                  1        n            ⁢                        ∑                      i            =            1                    n                ⁢                  "LeftBracketingBar"                                    x              ⁢                              xe2x80x83                            ⁢              i                        -                          x              _                                "RightBracketingBar"                      ,
where xi represents a measurement point value of each measurement point, {overscore (x)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 19)
A method as described in structure 18, wherein the microwaviness has a microwaviness period between 2 xcexcm and 4 mm.
(Structure 20)
A method as described in structure 18 or 19, wherein the modulation is evaluated by measuring a modulation resulting from the microwaviness of the surface of the substrate.
(Structure 21)
A method as described in structure 20, wherein the modulation is evaluated by measuring a waveprofile in a range between 1/50 and 1/4 of a microwaviness period of the microwaviness.
(Structure 22)
A substrate for a magnetic recording medium, the substrate having a microwaviness average height Raxe2x80x2 determined in accordance with a method described in any one of structures 18 through 21.
(Structure 23)
A substrate for a magnetic recording medium, the substrate having a predetermined microwaviness maximum height wa determined by:
measuring a microwaviness maximum height wa by the use of a contactless laser interference technique for measurement points in a measurement region on a principal surface of the substrate;
carrying out a touch-down-height test for a magnetic recording medium comprising the substrate and at least a magnetic layer formed thereon to obtain a tough-down-height;
comparing the microwaviness maximum height wa and the touch-down-height to obtain a correlation therebetween;
determining, with reference to the correlation, the predetermined microwaviness maximum height wa such that the tough-down-height has a desired value;
the microwaviness maximum height wa representing a difference value between a highest point and a lowest point of a measurement curve in all measurement points of a measurement area.
(Structure 24)
A method of controlling the surface of a substrate for a magnetic recording medium, comprising the steps of:
measuring a microwaviness maximum height wa by the use of a contactless laser interference technique for measurement points in a measurement region on a principal surface of the substrate;
carrying out a touch-down-height test for a magnetic recording medium comprising the substrate and at least a magnetic layer formed thereon to obtain a tough-down-height T;
comparing the microwaviness maximum height wa and the touch-down-height to obtain a correlation therebetween;
determining, with reference to the correlation, the predetermined microwaviness maximum height wa such that the tough-down-height has a desired value;
the microwaviness maximum height wa representing a difference value between a highest point and a lowest point of a measurement curve in all measurement points of a measurement area.
(Structure 25)
A method as described in structure 24, wherein the microwaviness maximum height wa is a value obtained by excluding from the measurement values xixe2x80x2 those values at points of abnormal protrusions.
(Structure 26)
A method as described in structure 25, wherein the microwaviness maximum height wa is a 95% PV value obtained by preparing a histogram of measurement point values xixe2x80x2 at all of the measurement points and extracting the measurement point values falling within a deviation of 95% in a distribution of all measurement point values in the histogram.
(Structure 27)
A method as described in any one of structures 23 through 26, wherein the measurement region of the microwaviness maximum height wa is smaller in area than a slider surface of a head slider of a recording and/or reproducing head.
(Structure 28)
A method as described in structure 26, wherein the principal surface of the substrate has a surface condition having a correlation between a microwaviness average height Raxe2x80x2 and the value obtained by excluding from the measurement values xixe2x80x2 those values at points of abnormal protrusions or between the microwaviness average height Raxe2x80x2 of the 95% PV value, the microwaviness average height Raxe2x80x2 being given by             Ra      xe2x80x2        =                  1        n            ⁢                        ∑                      i            =            1                    n                ⁢                  "LeftBracketingBar"                                    x              ⁢                              xe2x80x83                            ⁢              i                        -                          x              _                                "RightBracketingBar"                      ,
where xi represents a measurement point value of each measurement point, {overscore (x)} representing an average value of the measurement point values, n representing the number of the measurement points.
(Structure 29)
A method as described in structure 28, wherein the surface of the substrate is controlled by the microwaviness average height Raxe2x80x2.
(Structure 30)
A substrate for a magnetic recording medium, the substrate having a microwaviness maximum height wa determined in accordance with a method described in any one of structures 23 through 29.